1. Field of the Invention
The present invention relates to a constant voltage generating circuit, and in particular, to a constant voltage generating circuit composed of a band gap reference circuit constructed on a semiconductor integrated circuit and which is effective in reducing a driving voltage and noise.
2. Description of the Related Art
FIG. 4 shows a conventionally well-known band gap reference circuit. The principle of the operation of this circuit utilizes the fact that a positive temperature characteristic is exhibited by the difference (ΔVBE) between the base emitter voltage (VBE) at a bipolar transistor PN21 having a negative temperature characteristic and the VBE at a bipolar transistor PN11 having a different emitter area (that is, N times as large as that of the bipolar transistor PN21). Thus, Formula 1 is realized in a circuit so as to obtain a flat temperature characteristic.
                              V          ⁢                                          ⁢          O          ⁢                                          ⁢          U          ⁢                                          ⁢          T                =                                            α              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢              V              ⁢                                                          ⁢              B              ⁢                                                          ⁢              E                        +                          V              ⁢                                                          ⁢              B              ⁢                                                          ⁢              E                                =                                                    α                ⁢                                                      κ                    ⁢                                                                                  ⁢                    T                                    q                                ⁢                                  ln                  ⁡                                      (                    N                    )                                                              +                              V                ⁢                                                                  ⁢                B                ⁢                                                                  ⁢                E                                      ≅                          1.2              ⁢                                                          ⁢              V                                                          (        1        )                κ: Boltzman constant    q: electron load    T: temperature    α: 1+R2/R1
If the ratio of the area of the bipolar transistor PN21 to the area of the bipolar transistor PN11 is about 1:8, α (the voltage gain of a differential amplifier OP1) is about 13.
In view of the voltage gain of the differential amplifier OP1, since a PNP bipolar transistor is connected to the differential amplifier OP1 via a diode, the impedance between VSS and an emitter is low. Furthermore, an emitter terminal is considered to be substantially grounded, so that the differential amplifier is equivalent to an amplifying circuit having input resistance R1 and feedback resistance R2. Accordingly, the gain is (R1+R2)/R1=1+R2/R1=α. Given that noise from the differential amplifying circuit OP1 in input equivalent is defined as Vn, the noise characteristic is about αVn in output equivalent. Likewise, the offset voltage at the differential amplifier OP1 in input equivalent is a times in output equivalent.
For example, the circuits shown in FIGS. 5 and 6 are known to reduce noise (refer to, for example, Japanese Patent Application Laying-open No. 8-44449(1996)). The circuits in FIGS. 5 and 6 differ from each other in that one of them uses PNP bipolar transistors, while the other uses NPN bipolar transistors but their essential operations are equivalent to each other. The operations will be described below with reference to FIG. 6.
NPN transistors (NP11 to NP1n, NP21 to NP2n) having different emitter areas (in the present example, the ratio of the areas is N:1) are connected to two input terminals (+, −) of the differential amplifier OP1. Moreover, n NPN transistors are connected in series. Then, a potential difference ΔVBE occurs per stage, so that with the n NPN transistors, a potential difference nΔVBE occurs between both ends of R1. If PMOS FETs (P61, P62) have an equal W (channel width)/L (channel length) size, an equal current flows through the respective series NPN bipolar transistors. A voltage VOUT is expressed as follows:VOUT=αnΔVBE+nVBE=n(αΔVBE+VBE)≅1.2 nV  (2)If this output is reduced to 1/n, a voltage of 1.2 V is obtained as in the case with the circuit in FIG. 4. In this case, α is almost equal to the α in FIG. 4.
The noise from the differential amplifier OP1 in input equivalent increases by a factor of α as in the case with the circuit in FIG. 4. Furthermore, an input/output gain is equivalent to that of the circuit in FIG. 4. Accordingly, if the output is multiplied by 1/n to obtain a voltage of 1.2V, the noise characteristic is 1/n compared to the circuit in FIG. 4. The use of the circuit in FIG. 6 reduces noise compared to the circuit in FIG. 4.
Similarly, another bandgap circuit is known to reduce noise (refer to, for example, FIGS. 1 to 3 in U.S. Pat. No. 5,796,244).
As described above, the circuits shown in FIGS. 5 and 6 are considered to be constant voltage generating circuits having a reduced noise characteristic. However, in this case, bipolar transistors must be stacked, and a voltage of (1.2×n) V must be generated and then multiplied by 1/n to obtain a voltage of 1.2 V. In this case, the circuit must be operated with a power supply voltage of (1.2×n) V or higher. Disadvantageously, it is difficult to simultaneously achieve a reduced voltage operation and reduced noise.
Furthermore, with a circuit such as the one described in U.S. Pat. No. 5,796,244, no feedback is provided by an output stage (a circuit detecting nΔVBE does not act as a feedback circuit). Consequently, changes in environment may preclude accurate outputs from being obtained.
Thus, the present invention is directed to providing a constant voltage generating circuit that solves the above problems.